Friday, 14 February 2014

How is energy being transferred in a conventional circuit? A new but coherent perspective.

I saw this videos some years ago and shared with my graduating class students even though it is not the usual way energy transferred in circuits is being taught and assessed.

Two days ago, I watched this video again while searching for videos for Quantum Physics. I was determined to find out how energy can be transferred outside the wire!

Few interesting concepts from my internet research, some was new to me before my research:

The battery (source) causes a different distribution of surface charges along the circuit, this indicate a slightly non-neutrally along short sections in the circuit. This is possible as there is dynamic equilibrium (analogous to a pressure gradient in fluid flow in pipes) in the circuit. Conventional teaching claims that the entire circuit is neutral because the net charge is zero, which is correct, but we incorrectly extend it to mean that every section of the wire must be neutral.

The distribution of surface charges result in (a) an electric field along the wire and (b) electrostatic field that extend radially from the wire.

The electric field along the wire is what we normally associate as the potential difference between two points in a circuit (remember that E = -dV/dr).

The magnetic field associated with the movement charges in the wire is perpendicular (tangential to "circles" around the wire) to the radial electric field due to the surface charges.

The cross product E x B gives the Poynting vector that gives the direction of energy flow/transfer. This is how energy is transferred through space outside the wire!

The following are some of the articles I have gathered the past two days:

A unified treatment of electrostatics and circuits, B. A. Sherwood and R. W. Chabay - a more detail development of the concepts compared to the previous Am. J. Phys. article. Possibly a guide for teachers who want to challenge their students' understanding of circuits.